Patients commonly suffer infections that go unrecognized and lead to sepsis because of the associated trauma, burns and invasive life support. Every hour between diagnosis and administration of the correct treatment decreases survival significantly. For example, in one study, the risk for in-hospital mortality increased by 9% for every hour of delay before the correct antibiotic regimen was administered. Garnacho-Montero, J. et al., Critical care 10, R111 (2006). Thus, the speed of pathogen diagnosis in a soldier with a blood-borne microbial infection can mean the difference between life and death. However, infections are challenging to diagnose in the field or in a hospital.
Sepsis is a clinical syndrome defined as the systemic response to infection. Although it is one of the leading causes of death in developed countries and is responsible for one in three to one in two deaths in hospitalized patients in the USA, no therapeutic nor diagnostic breakthrough has occurred since the discovery of antibiotics and the improvement in supportive measures. The diagnosis of sepsis is still highly empirical and is defined as a systemic inflammatory response syndrome (SIRS) caused by an infection.
In general, existing diagnostics of infection rely on two concepts: direct diagnosis and indirect diagnosis. Direct diagnosis is based on evidencing the presence of a pathogen in a clinical sample. For example, direct examination, which is specific but has a low sensitivity, can allow direct observation of the pathogen through specific coloration using differential dyes. Blood culture is the gold standard to prove infection. However, it is time consuming (e.g., about 1-7 days), and only 5% to 15% of the all cultures drawn for any reason are positive, and half of patients with septic shock have negative blood cultures and no microbiological documentation of the infection. Murray and Masur, Critical care medicine 40, 3277 (2012). Tuberculosis cultures can particularly be time consuming, ranging from 1-60 days.
Nucleic acid-based detection generally relies on polymerase chain reaction (PCR). However, it can be susceptible to false negatives from failure to extract bacterial DNA, competition from host DNA and inhibition by matrix components and/or to false positives due to its high sensitivity to contaminating DNA, and has failed to improve the management of sepsis patients. Microbial antigen based detection is generally based on the detection of microbe specific antigens by monoclonal antibodies. The existing microbial antigen-based assays can detect Legionella pneumophila serotype 1 in urine, Streptococcus pneumoniae in urine and fungal carbohydrates in blood.
Indirect diagnosis is generally based on the detection of the host response to infection. For example, serological assays are typically based on the detection of host antibodies directed against microbial determinants and allow specific identification of the agent when unique microbial epitopes can be used for the capture of unique host antibodies. Thus, diagnosis is often retrospective and can be used for the diagnosis of chronic infections (e.g., Lyme disease and syphilis, viral infections). Proteomic biomarkers are host proteins that can show marked variations in infected patients. Proteomic biomarkers can be sensitive but existing target markers (e.g., C-Reactive Protein, Procalcitonin, IL-1, IL-6, THF) are not specific for infection and general stresses such as recent surgery, trauma, neoplastic diseases or autoimmune disorders can also affect the same markers used to detect infection. Thus, there is currently no test allowing the diagnosis of infection in the SIRS patient population.
Sepsis diagnosis is therefore dependent on the combination of a clinical suspicion of infection and general clinical and biological criteria such as heart and respiratory rates, temperature, hyperglycemia or white blood cells counts. Patient stratification schemes distinguish severe sepsis based on acute organ failure documented by simple clinical and biological variables (lactates, platelets, bilirubin, creatinine, capillary refill or urine output) and septic shock associated with refractory hypotension (BP<90/60) despite adequate fluid resuscitation and/or a serum lactate level ≧4.0 mmol/L. Multiple biomarkers have been evaluated to document sepsis and predict patient evolution to enhance medical care but are not integrated in the diagnosis or staging of sepsis. Most of these biomarkers are host proteins involved in the inflammatory response or associated with SIRS. No marker unquestionably distinguishes sepsis from SIRS caused by burn, trauma or surgery.
Severe sepsis and septic shock are rapidly evolving conditions in which delayed administration of empiric antimicrobial therapy is recognized to increase mortality by the hour. The multiple days required to obtain microbiological documentation makes empirical treatment an absolute necessity. The emergence of extensively drug resistant (XDR) bacteria challenges current therapeutic protocols for sepsis patients since a stereotyped empirical antimicrobial agents protocol is less likely to provide effective coverage of all the pathogens commonly encountered in any given clinical setting. Most concerning is the spread of carbapenem resistant Gram negative bacteria that commonly require the association of up to three molecules, the resistance to any agent being likely to generate resistance to the other molecules. As the requirement for antimicrobial documentation becomes more pressing, the current laboratory practices cannot provide microbiological documentation or antimicrobial susceptibility in the clinically relevant timeframes. It has been the clinical practice to document some infections by a “therapeutic trial”. When suspecting an infection without microbiological documentation, physicians have been testing the hypothesis of the bacterial etiology of a clinical disorder by providing antibiotics and monitoring the clinical improvement of the patient under the treatment. A clinical improvement equated to a diagnosis of infection. In the absence of improvement over the duration of the “trial”, the treatment was changed to encompass different pathogens or the infectious etiology was ruled out.
Moreover, while guidelines advise the revision of the antimicrobial regimen every 24 hours, the adequacy of the antimicrobial regimen is not necessarily apparent before several days and the use of biomarkers such as procalcitonin (PCT) do not yield the expected improvement in patient care. To this day, microbiology laboratory tests have had little impact on the management of sepsis patients.
The diagnosis of sepsis is currently purely clinical and the definition of sepsis is “SIRS in the context of infection.” However, the generic symptoms of systemic inflammatory response syndrome (SIRS) (e.g., fever, chills, rigor, high heart/respiratory rates) are generally observed regardless of infection. Thus it is difficult to diagnose infections because local infections and blood-borne infections that can result in, e.g., sepsis, often produce similar generic symptoms (e.g., fever, chills, rigor, high heart/respiratory rates). Accordingly, there is an unmet need to develop an extremely rapid (<1 hr) sepsis diagnostic that can detect the presence of systemic infections in blood samples from patients even when blood cultures are negative and the ability to monitor the efficacy of antimicrobial treatment is key for further antibiotic susceptibility testing development.